The present invention relates to an etching method and an etching apparatus for etching inorganic materials, such as metal, metal oxide, and the like, a manufacturing method for manufacturing a semiconductor device, and a semiconductor device.
Etching techniques together with lithography techniques support micro-processing techniques in the process for manufacturing semiconductors. Conventionally, there has been no other ways than physical removal based on polishing, with respect to substances on which a determination has not yet been made as to etching removal with use of a solution. For example, metal oxide (SrRuOx) which is an oxide of strontium (Sr) and Ruthenium (Ru) is one of materials used for FRAM, DRAM, or the like. Since no method for etching has been proposed as to this kind of metal oxide, only the physical removal has been carried out.
Meanwhile, H2O2 is known as a solution for conventional cleaning. To remove particles and organic materials, NH4OH/H2O2 is mainly used and a cleaning solution containing this material is alkaline. To remove metal impurities, HCl/H2O2 is mainly used and a cleaning solution containing this material is acid. These cleaning solutions are widely used before and after a step in which impurities are involved in the semiconductor process, and the cleaning using them is generally called RCA cleaning. Also, two kinds of cleaning solutions described above are used most generally and are called SC1 and SC2, respectively.
For example, as described above, cleaning with water containing ammonium hydrogen peroxide has been used in particle cleaning. In the system for removing particles, it is considered that particles are lifted off during etching of a silicon substrate using alkaline solution, and the particles and the surface of the silicon substrate are set to an equal potential, thereby inducing mutual repulsion which hinders re-sticking.
Also, in case of removing organic substances by the RCA cleaning, cleaning with H2SO4 is carried out. However, there are problems concerning scattering of sulfuric acid ion into a clean room and recycling of waste liquids. Hence, ozone-added ultrapure water technique has been proposed. This cleaning method is used as a strong oxidation agent for subjecting an ozone gas to oxidation decomposition. This method is advantageous in that organic substances can be effectively removed by a slight amount of ozone gas and that a waste liquid needs not be recycled since an ozone gas changes into oxygen due to autolysis. If ozone water is thus used for cleaning, the ozone water has a concentration of about 5 to 20 ppm at the room temperature, in general.
Further, HCl/H2O2 cleaning and HF/H2O cleaning has been carried out as a method of cleaning metal impurities. By the HCl/H2O2 cleaning, impurities in the native oxide film cannot be removed although metal impurities sticking to the surface of the silicon substrate and metal impurities sticking to the surface of the native oxide film can be removed. Therefore, a step of removing a native oxide film with use of HF/H2O2 is carried out additionally. In the method of removing the metal impurities, cleaning with use of ozone water has been proposed like in the above case of removing organic materials.
As has been explained, ozone water is used for cleaning of organic impurities on the wafer surface with utilization of an oxidation force of ozone, and cleaning and sterilization of organic impurities in pure water. With respect to solution of organic materials, the ozone water is used only from the viewpoint of cleaning.
In contrast, it has been recently attempted to use ozone water for etching of organic films such as a resist and the like. In case of etching of an organic film, the concentration of ozone in ozone water must be set to about 100 ppm in order to ensure an effective etching rate.
However, even in this case, ozone water is used for the purpose of etching an organic film but is not used for etching and removing inorganic materials such as metal and metal oxide films.
Meanwhile, a CMP (Chemical Mechanical Polishing) method has been widely used as a method of polishing and flattening the surface of a semiconductor substrate. This CMP method is a method of flattening the substrate surface with use of both the chemical operation and the physical operation. By using (NO3)2CeNH4 as an oxidation agent to be added to slurry, it is possible to flatten the surface of an SrRuOx film, which is a metal oxide. However, in case where this material is not used for CMP but is used for chemical etching, (NO3)2CeNH4 requires a concentration of several % order in order to ensure an effective etching rate (10 to 100 nm/min). However, (NO3)2CeNH4 is an oxidation agent which causes autolysis and is therefore unstable, so that it is very difficult to maintain a high concentration as a liquid for slurry. Also, since acid-alkaline waste liquid processing is required, this oxidation agent is difficult to treat as an etching solution.
As described above, it is difficult for a conventional etching technique to etch metal oxide materials such as an SrRuOx and the like used for FRAM, DRAM, and the like. Consequently, there is no other way than carrying out physical removal based on polishing. Even in case of carrying out polishing, since the several % order cannot be stably maintained for (NO3)2CeNH4, (NO3)2CeNH4 must be dissolved as an additive agent immediately before etching, and thus a liquid for slurry must be prepared. It is thus very difficult to use this method for a semiconductor manufacture process. In addition, a discussion has been made as to use of ozone water for cleaning and etching of an organic film which are carried out before or after the step in which particles and metal impurities of dry etching or the like are generated. However, no consideration has been made as to application to metal and metal oxide films such as an SrRuOx film and the like.
An object of the present invention is to provide an etching method, an etching apparatus, and a method of manufacturing a semiconductor device, which are capable of easily removing a film made of an inorganic substance which cannot conventionally be etched without physical removal and without polluting the environment.
To achieve the above object, according to a first aspect of the present invention, an etching method comprises the steps of: preparing a substrate having a surface on which a film made of an inorganic substance is formed; and wet-etching the film on the substrate by oxidation-reduction reaction involving oxygen, with use of an etching solution containing an oxidation agent which has an oxidation-reduction potential of 2V or more measured at 25xc2x0 C., 1 atm.
The oxidation agent captures and releases electrons and generates oxygen, in the oxidation-reduction reaction.
It is desirable that the inorganic substance is difficult to dissolve in water and the oxidation-reduction reaction generates an easily water-soluble or volatile substance from the inorganic substance.
It is also desirable that the inorganic substance is a metal or a metal compound and the inorganic substance is strontium ruthenium compound which is decomposed into an easily water-soluble compound and a ruthenium compound by the oxidation-reduction reaction.
The etching solution is desirably ozone water having a concentration of 5 ppm or more when the etching solution reaches the film.
Desirably, in the step of wet-etching the film, the substrate having the film made of strontium ruthenium compound is rotated at a speed of 100 rpm or more and the ozone water is directly injected onto the substrate.
Also desirably, in the step of wet-etching the film, the substrate having the film made of strontium ruthenium compound is rotated at a speed of 1000 rpm or more and the ozone water is directly injected onto the substrate.
The etching solution is desirably ozone water having a concentration of 100 ppm or less when the etching solution reaches the film.
According to a second aspect of the present invention, an etching apparatus comprises: a substrate rotation mechanism for rotating a substrate to be processed, which has a thin film made of an inorganic substance, at a rotation speed of 100 rpm or more; and an ozone water supply mechanism provided on the substrate rotation mechanism, for supplying ozone water having a concentration or 5 ppm or more onto a surface of the substrate to be processed, thereby to etch the substrate.
It is desirable that the ozone water supply mechanism supplies ozone water having a concentration of 5 ppm or more when the ozone water reaches the surface of the substrate to be processed.
Desirably, the ozone water supply mechanism supplies the ozone water to a center of the substrate to be processed or vicinity thereof.
Also desirably, the substrate rotation mechanism rotates the substrate at a rotation speed of 1000 rpm or more.
According to a third aspect of the present invention, a method of manufacturing a semiconductor device comprises steps of: forming a first metal compound film on a semiconductor substrate, the first metal compound film being formed of metal elements, at least one of which has compound which is difficult to dissolve in water; forming a second metal compound film on the first metal compound film, the second metal compound film being formed of metal elements, each of which makes an easily water-soluble or a volatile compound; forming a protection film having a desired aperture, on the second metal compound film; and etching the second metal compound film exposed from the aperture, by a solution of water containing ozone.
The second metal compound film is desirably strontium ruthenium compound which is decomposed into an easily water-soluble or a volatile strontium compound through the etching step.
It is desirable that the solution of water containing ozone is ozone water which has a concentration of 5 ppm when the ozone water reaches a surface of the second metal oxide film.
Desirably, in the etching step, the substrate having the second metal compound film made of strontium ruthenium compound is rotated at a speed of 100 rpm or more and the ozone water is directly injected onto the substrate.
According to a fourth aspect of the present invention, a semiconductor device comprises: an insulating layer having a trench; a first metal compound layer formed on an inner surface of the trench; a second metal compound layer which is formed on the inner surface of the trench with the first metal compound inserted therebetween and extends on an upper surface of the insulating layer; a third metal compound layer which fills inside of the trench with the first and the second metal compound layer inserted therebetween and extends on the upper surface of the insulating layer with the second metal compound layer inserted therebetween; and a mask layer formed on the third metal compound layer and covering at least the third metal compound layer.
An outer edge of the third metal compound layer is formed inside an outer edge of the mask layer, and a distance between the outer edge of the third metal compound layer and the outer edge of the mask layer is set to be 0.3 xcexcm or less.
The third metal compound layer is desirably formed of a strontium-ruthenium compound layer.
According to the present invention, a substrate to be processed which has a film made of an inorganic substance is wet-etched with use of an oxidation agent having an oxidation-reduction potential of 2V or more measured at 25xc2x0 C., 1 atm, with oxygen involving in oxidation-reduction reaction. As a result, an inorganic film can be easily etched without using physical removal conventionally used.
In addition, by rotating the substrate to be processed at 100 rpm or more during etching, it is possible to enhance the effect of mixing ozone water and reaction products (RuOx and SrOx) at the interface where the etching proceeds. As a result, it is possible to remove substances generated at the interface by oxidation-reduction reaction and to introduce a new oxidation agent to the interface, so that etching can be proceeded sufficiently.
In particular, in case of etching a film made of an inorganic substance which is difficult to dissolve in water, those inorganic substances that cannot be dissolved in a normal etching solution can be etched easily by converting those inorganic substances into water-soluble or volatile substances through oxidation-reduction reaction.
In addition, since ozone water is used as an oxidation agent, ozone changes into oxygen due to autolysis through the oxidation-reduction reaction. It is therefore unnecessary to collect waste liquids.
Further, in case where an SrRuOx film is used as an inorganic film and a BaSrTiOx film is formed as a lower layer below the inorganic film, only the SrRuOx film can be selectively etched. That is, the SrRuOx film is decomposed into SrOx and RuOx by oxidation-reduction reaction, but all of Ba, Sr, and Ti are not changed into water-soluble compound (oxide) in a neutral range. Therefore, the BaSrTiOx film cannot be etched practically. As a result, only the SrRuOx film can be selectively removed.
Accordingly, a layered film consisting of an SrRuOx electrode and a BaSrTiOx insulating film can be used for a memory such as FRAM, DRAM, or the like, so that a device using a ferroelectric material having a perovskite structure can be manufactured easily at low costs without polluting the environment.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.